John Collins says he expects some players to leave Celtic in the summer after admitting it's difficult to keep all of the club's midfielders happy.

It comes after Norwegian teenager Kristoffer Ajer became the 17th midfielder to be added to Ronny Deila's squad.

As many as 14 are available for selection with only Ajer, and youngsters Aidan Nesbitt and Liam Henderson, who are on loan to Partick Thistle and Hibernian respectively, not returning to the club until the summer.

The assistant manager believes that while strength and depth are normal at every club, some may not want to stick around and fight for their place in the starting line-up.

"I think come the end of the season there will be players who move out but at this moment in time we're a big squad and everyone is fighting for jerseys," Collins said.

"You can't keep them completely happy but everybody would be telling a lie if they said everybody's happy at a football club.

"The 11 starters are very happy, the substitutes who are a little bit unhappy and the ones who are sitting in the stand who aren't happy.

"That's the same the world over, every club in the world, but the most important thing is the players realise that to get into the team they have to train well and keep focused."

As one of several midfielders vying for a spot in the starting line-up, Stuart Armstrong says he doesn't have an issue with the competition he faces.

"I think it's the reality of football, there's competition everywhere and you just have to accept it," he said.

"It's a team and everyone wants the team to do well. Of course in training everyone is competing for a spot in the team, to get as close to that as possible.

"There's a good atmosphere in the training ground and I wouldn't say there's any negativity towards that."

Stuart Armstrong has admitted his former club's hopes of remaining in the Scottish Premiership are slim but says they must remain optimistic going into the final stages of the season.

Dundee United sit rock-bottom of the table, 13 points adrift of Kilmarnock with 13 games remaining, but the midfielder, who left the club a year ago, says the Tannadice club have to be positive.

"Their future isn't looking good in this league but you've got to remain optimistic and hope they can finish the league strongly and if they do go down they'll come back as soon as possible," he said.

"I still know a few of the boys there, what they're capable of and what good professionals they are so I think it's all circumstantial.